A conventional image intensifier is illustrated in FIG. 1, which is a longitudinal section. It comprises a glass faceplate 1 on which a photocathode 2 is laid down, a micro-channel plate electron multiplier 3 and a phosphor screen 4 on a glass substrate 5. The glass faceplate 1 and substrate 5 form the end windows, of a vacuum envelope in which the other elements are contained. In operation an optical image is focussed on the photocathode 2 by an external lens, causing electrons to be emitted. The electrons are accelerated to the electron multiplier 3 where they are increased in number by secondary emission at the channel walls. The secondary electrons are then accelerated towards the phsophor screen 4 by a potential difference set up between it and the electron multiplier 3, producing an optical image corresponding to the image focussed on the photocathode 2 but of very much greater intensity. By increasing the potential difference, a typical value being several thousand volts, the intensification produced is increased. The flat surfaces of the photcathode 2, electron multiplier 3, and phosphor screen 4 must be closely spaced and parallel to ensure good definition in the final image. To achieve this without discharge breakdown occurring between electrical connections 6 to the photocathode 2, electron multiplier 3 and phosphor screen 4, the glass faceplate 1 and glass substrate 5 have central raised portions extending towards the electron multiplier 3. Thus the electrical connections 6 are spaced much further apart than would otherwise be the case.
In such an image intensifier light incident at the periphery of the glass faceplate 1 may undergo reflection within the faceplate 1, as shown at 7, resulting in spurious signals. This may be reduced by employing a shielded faceplate consisting of a central transparent region surrounded by light absorbing material.
In a previous method of manufacturing such a shielded faceplate a cylindrical clear glass rod 8 is inserted into a black glass tube 9, as shown in FIG. 2. The rod 8 and tube 9 are surrounded by a tubular clear glass envelope 10 which is evacuated. The temperature is then raised until the glasses fuse (FIG. 3) producing a solid cylindrical block. This is sliced transversely to its longitudinal axis X. One of the slices is shown in FIG. 4. The slice is then machined to the required shape shown in FIG. 5, to produce a shielded faceplate having a clear glass core 11 surrounded by an outer region of black glass 12.
This method has a number of disadvantages. The machining required is expensive and, in the case of obtaining the initial black glass tube, is extremely difficult, since its inner surface must be polished to a high quality. In addition, distortions are introduced during the heating process, resulting in a loss of concentricity. Also the parallel sides of the clear core 11 cause vignetting, or fading of light entering the faceplate at the periphery of the core 11.